An electronic projector (beamer) converts electronic image information signals or data into optical images and projects them onto a suitable projection surface, for example a projection screen. The beamer receives as input signal a video signal from the video card of a computer or any other analog or digital signal source, for example a video recorder or a DVD player. If the input signal is not already present in digital form, it is digitalized in the beamer and broken up into the color components R, G and B. An optical modulator is controlled with the color signals R, G and B after various adaptations and corrections. The modulator is read out by way of illumination optics and projected by way of projection optics onto a projection surface. Today's beamer technologies use as optical modulators either TFT-LCD's (thin film transistor-liquid crystal displays), polysilicone-LCD's or a matrix of moveable micro-mirrors, for example the DMD-chip (digital micro-mirror device) from Texas Instruments. Optical modulators are generally also referred to as light valves.
The optical system of beamers is normally optimized for maximum efficiency of the exposure and projection optics. It is thereby the goal to produce on the projection surface an image of the highest possible light intensity in order to achieve the highest possible visible contrast in rooms which are not darkened.
The color reproduction quality of simple beamers is relatively low despite careful calibration (brightness, contrast, color temperature). Compared to a monitor image, clearly visible color tone shifts occur. Furthermore, the colors appear insufficiently saturated because of the construction for maximum light intensity. Modem beamers are therefore equipped with a transformation stage (color space converter) which transforms the supplied digital or digitalized input signal into the color space of the beamer and thus significantly improves the color trueness of the reproduction, whereby the limits are then given essentially only by the color space (gamut) reproducible by the beamer and the reflective properties of the projection surface. The transformation parameters used in the color space converter describe the required corrections.
Color management and color management systems are principally known and are generally used in digital color reproduction processes. A comprehensive and clear analysis of the background, technologies and applications of color management systems is found in the publication “Postscriptum on Color Management, Philosophy and Technology of Color Management” by the authors Stefan Brües, Leanne May and Dietmar Fuchs, published by the company Logo GmbH, a company of the Gretag-Macbeth Group in August of 1999. A further discussion of color management is found, for example in chapter 17 “Device Independent Color Imaging” of the book “Color Appearance Models” of Mark D. Fairchild, first edition, published 1997 by Addison Wesley.
A color management system allows a true to color reproduction of an image original in a digital process which can include an input medium (for example a scanner or digital camera) and several output media (for example monitors and printers). The original is digitized with the input medium and the RGB values of each image pixel are stored in a data file. The RGB image can be reproduced on a monitor or by way of a beamer or recalculated for output on a printer in the print colors CMYK (cyan, magenta, yellow, black). The true to color reproduction is achieved by the application of a device calibration and device profiles. The above mentioned color space converter essentially realizes a device profile, as is known from color management, in hardware.
In the device calibration, the base adjustments of the device are selected as optimal as possible for the desired reproduction. For a beamer this includes, for example, adjustment of the contrast, the brightness, the white and black point as well as the color temperature according to the desired type of illumination. The calibration can be optimally carried out in a generally known manner by way of color measurement devices and especially made reproducible.
A device profile describes the color space of an input or output device, in that it assigns to the device specific color vales (for example RGB or CMYK) absolute color values (XYZ or CIE Lab L*a*b*) according to the specifications of the CIE (Commission Internationale de l'Eclairage) or other not device specific color values (for example sRGB). A device profile typically consists of various color value replacement tables (lookup tables) and coefficients of a color space transformation matrix. The device profile is strongly dependent on the device calibration. In order to successfully carry out color management, it is therefore a prerequisite that the calibration of all input and output devices involved does not change. It must be kept in a constant condition by regular control and readjustment.
For the true to color image reproduction, the color management system combines the profiles of the input and output medium and in that way obtains a transfer characteristic. At the same time, the CMS carries out a mapping of the color space of the input device onto the color space of the output device. This mapping ensures that all colors can be reproduced on the output medium. The mapping, which means the resealing of the color spaces, is carried out according to different reproduction criteria, the so-called rendering intents. One thereby distinguishes between the modes “perceptual” (equal color impression in the image), “relative colorimetric”, “absolute colorimetric” and “saturation”, which are defined in document ICC-1: 1989-09 of the International Color Consortium.
For the profile assembly, the color space of the input and output devices must be measured with specialized color measurement devices or by way of colorimetrically calibrated samples, so-called test charts. For optimal results, the color measurement technology used should be based on the spectral measurement (spectrolphotometry). This enables the recognition of metamerism and fluorescence, as well as the correct evaluation of different illumination types.
The color management and most of all the color measurement technology for the beamers is more involved compared to monitors, which as output medium are also controlled with RGB signals. The reason therefor lies in the fact that with the beamer the color impression depends not only on the device itself, but also on the reflection and diffusion characteristics of the projection surface (for example screen), the spectral distribution and the brightness of the ambient light in the room, as well as perception and experience determining factors of the human observer, for example, the degree of adaptation of the eye. These factors must be metrologically captured and taken into consideration in the color management system. These factors are also responsible for the fact that a base calibration and profiling carried out during the manufacture of the beamer does not enable optimal color reproduction. Ideally, the calibration and profiling must be carried out at the user in regular intervals and repeated, so that changes and aging processes in the optical system of the beamer can also be compensated.
A beamer with an integrated color management system is described in EP-A-1 150 159. The beamer is equipped with an integrated or externally connected color measurement device, which is constructed as a tri-stimulus color measurement device or preferably as a spectrophotometer. The color measurement device receives the light remitted (reflected and scattered) by the projection surface within a measurement region (measurement spot) and produces the corresponding absolute (standardized) color measurement values X, Y and Z (or values derived therefrom). A profile producing function and a calibration function are further provided in the beamer in addition to the normal components and functional units, which cooperate with the color measurement device and receive its color measurement values as input data. The color measurement values produced by the color measurement device are then analyzed in the beamer for the automatic first and recalibration and for the calculation of the device profile (adjustment of the color space converter). Since the color measurement device measures the light coming back from the projection surface, the profile produced on the basis of the color measurement values describes the whole transfer characteristic of the beamer starting from the analog or digital video data stream supplied to the beamer, through the beamer itself and the projection surface, up to the eye of the observer.
The beamer described in EP-A-1 150 159 requires that the beamer be originally equipped with the required components to cooperate with the color measurement device and to be able to analyze its measurement values in the mentioned manner. However, most of the commercially available beamers are not equipped in this way.